1. Field of the Invention
The invention relates to an oil suction system, and in particular to an oil suction system provided in a lower part of an automatic transmission of a vehicle.
2. Description of the Related Art
In a vehicle, such as an automobile, including a power transmitting system, such as an automatic transmission, incorporating a hydraulic circuit, al oil suction system is installed which includes an oil pan mounted on the bottom of the power transmitting system, in order to improve the capability of collecting oil to be supplied to and discharged from the hydraulic circuit. In the oil suction system, the oil pan is often required to have a shallow depth so as to ensure a sufficient ground clearance of the vehicle, while it is necessary to prevent air from being drawn up even if the liquid level of the oil near a suction port is lowered.
For example, Japanese Patent Application Publication No. 10-121933 (JP-A-10-121933) discloses an oil suction system for use in the engine, in which a pair of partition walls that are opposed to each other are provided in the vicinity of the interface of two casings, so as to form an oil suction space having two oil chambers with different amounts of oil stored therein, and a suction port is slightly inclined at the inlet of the oil suction space so that the larger amount of oil is drawn from one of the above-indicated oil chambers in which the larger amount of oil is stored.
Also, Japanese Utility Model Application Publication No. 5-90020 (JP-U-5-90020) discloses an oil suction system of an automatic transmission in which an oil pan having a reduced depth is often used, a strainer is supported onto a valve body at a position remote from a suction port such that the strainer is rotatable about the position (support) in a horizontal plane, and a weight is mounted at the suction-port side of the strainer, so that the suction port moves in a direction in which the oil in a reservoir moves during acceleration, deceleration, turning or braking of the vehicle, thereby to prevent drawing of air into the strainer.
Also, Japanese Patent Application Publication No. 2007-56677 (JP-A-2007-56677) discloses an oil suction system of an engine, in which a strainer is located close to the inner surface of the bottom of an oil pan and extends in the longitudinal direction, and an opening (suction port) formed in a rear end portion of the strainer is inclined such that the distance between the opening and the inner bottom surface of the oil pan increases from the front end to the rear end of the opening. This arrangement is developed in view of the situation where the oil moves to the rear side of the oil pan during uphill running or upon rapid acceleration, mid the liquid level is lowered at the front side and is raised at the rear side.
In the oil suction systems as described above, however, the oil chambers are formed by providing the mutually opposed partition walls at the interface of the cases, or the suction port is made movable in the direction of movement (shift) of the oil, thus making the construction of the system complicated and increasing the cost of manufacture of the system.
In the system in which the rear end portion of the strainer extends along the inner bottom surface of the oil pan, the suction port is located in a longitudinally middle portion of the oil pan, and the rear end portion of the strainer needs to be supported onto the oil pan, which makes the construction of the system complicated. Furthermore, the oil suction system thus constructed cannot be expected to provide a sufficient effect of preventing drawing of air if it is employed as an oil suction system of an automatic transmission in which the depth of the oil pan is further reduced, and the strainer assumes a flattened shape extending in a horizontal plane.
Namely, in recent automatic transmissions, the amount of oil stored in the transmission needs to be reduced for an improvement in the efficiency particularly at high oil temperatures and for reduction in weight. If the amount of the oil stored is reduced, however, a low-oil-level portion (in which the amount of oil drawl into the strainer is larger than the amount of oil supplied to this portion) appears in the oil pan when the transmission is inclined at a low temperature (e.g., at about 0° C.), and drawing (suck-up) of air is likely to occur at the low-oil-level portion. If the oil storage capacity in each direction of the case differs when the transmission is inclined frontward, rearward, leftward and rightward, the level of the liquid surface (the liquid level) close to the suction port of the strainer upon inclination varies greatly even with the same oil amount. Furthermore, the oil supplied to the oil pan does not evenly or uniformly flow into the suction port, because of the layout of the interior of the oil pan, and a portion having a small amount of oil appears in the oil pan. For these reasons, and others, drawing (suck-up) of air into the oil pump occurs even where the suction port that is located in the central portion of the oil pan and is open backward while being inclined is provide at the rear end of the strainer that extends in the longitudinal direction. As a result, abnormal or unusual sound due to entry of air into gears may occur in an oil pump constituted solely by a gear pump, or poor pressure regulation may occur due to entry of air into a valve body.